Aluminum and aluminum base alloys have become important materials in all engineering fields and all the common methods of joining are applied to aluminum and its alloys. With the exception of bolting and riveting and regardless of the considerable differences between them all common joining methods involve the same fundamental property of adhesion, namely, that of bringing the surfaces to be into sufficiently close contact for the interatomic forces to be effective. In pressure welding, and to a lesser extent in resistance welding, the joints are made entirely between solid phases: in fusion welding, the joints are consummated entirely by liquid phases: in brazing and soldering both solid and liquid metals are involved, while when plastic adhesives are used solid metal is joined by a liquid or semi-liquid organic material.
Aluminum was generally regarded as a difficult metal to join by any means other than mechanical. Recent research, however, has led to a better understanding of the properties of the metal which do and/or do not promote joining aluminum and today the results of the aforesaid research have led to the proliferation of such methods.
The methods used to join aluminum are basically the same as those used to join other metals except that modifications in technique have been used to take into account the effect the properties of aluminum. The properties that work against the joining of aluminum by exactly the same techniques that are used to join other metals are: thermal properties; the oxide film; gas reactions; and metallurgical characteristics. Pure aluminum melts at 659.degree. C. and the alloys melt over a range of from about 530.degree. C. to about 650.degree. C. depending upon their composition. When fusion welding is used experience with a particular alloy is necessary because no color change occurs on heating. Additionally, a considerable heat input is required to raise the mating surfaces of aluminum to its joining temperature because the specific and latent heats of aluminum are high and because the thermal conductivity of aluminum is high and varied. The thermal conductivity of aluminum varies from approximately five times that of mild steel in the case of the pure metal to about three times that of mild steel for the alloys.
Perhaps the single most important factor to be taken into account when making satisfactory aluminum joints is the presence of the oxide film which covers the surface of all aluminum-base materials. The rate of oxidation of aluminum is extremely high and the oxide formed does not melt until temperatures above 2000.degree. C. are reached and it is also insoluble in liquid and solid aluminum. This oxide layer on pure aluminum is sufficiently continuous and tough to prevent metal to metal contact even when the metal melts. Oxide fils on aluminum-base materials are porous and retain significant amounts of substances such as grease, dyes and other compounds with which they may become impregnated. For this reason it is necessary to either remove the oxide film completely before or during the joining operation or to ensure that the surface film is uniform in thickness and characteristics over the surface being joined.
It is well known that liquid aluminum absorbs considerable volumes of hydrogen which is less soluble in the solid metal and solidification of a melt into which much hydrogen has been dissolved during welding will result in a porous and consequently weak joint. Porosity in a weld could also result from the dissolution of water vapor and constituent gases from the welding flame in the molten metal at the point of joining.
Soldering aluminum is not yet as generally practical as are welding and brazing, but it has been successfully used to join aluminum cable sheaths and to fill dents in damaged aluminum car panels. Several methods of soldering have been used in the past. These methods all require a high degree of experience and skill to be successfully practiced. One such method involves rubbing a previously cleaned surface with a hot soldering iron and stick solder to first remove the surface oxide layer and then to tin the surface with melting solder before the surface can reoxidize. The friction or rubbing method as this is called requires skill and experience and must be used in combination with chemical cleaning. Another method known as reaction soldering makes use of the reaction between aluminum and zinc chloride whereby volatile aluminum chloride is produced and zinc is deposited on the aluminum surface to provide a suitable base for subsequent soldering. Another soldering method involves immersing aluminum in a bath of molten solder and using ultrasonic waves transmitted through the molten solder to shatter the oxide film thereby allowing the aluminum to be effectively wet by the solder.
One problem with soldered aluminum joints results from the possibility of solder penetrating the aluminum at its grain boundaries and causing some risk of embrittlement and cracking.
During the past several years synthetic resin adhesives have begun to be used for joining like metals, dissimilar metals and metals to non metals. The aerospace industry has led the way in the development of this method for aluminum. The primary characteristic of a plastics joining process is the wetting of a cleaned surface by a liquid or solid of low melting point followed by curing which changes the liquid to a hard infusable resin which will adhere strongly to the metal. Good adhesion depends upon thorough wetting of the surfaces to be joined by the adhesive. Wetting demands thoroughly cleaned and degreased surfaces but in this process the oxide film serves a useful base for adhesion. Because corrosion of the metal cannot be tolerated the types of plastics acceptable for use as the adhesive are limited. The processes used to bond aluminum are divided into two classes based upon the method of curing, which in both cases, is carried out at elevated temperatures. One class requires the application of appliciable pressure during curing and the other requires only sufficient pressure to hold the surfaces being joined together and in close contact during curing. One such method uses a phenol formaldehyde resin which is either brushed or sprayed onto clean aluminum to wet the metal and a polyvinyl formaldehyde powder is then sprayed onto it to give toughness and strength after curing. After a period of open exposure before increasing temperature and pressure the pressure is increased to about 100 tons per square inch at a temperature of 140.degree. to 150.degree. C. for 15 to 20 minutes. If the temperature is higher the curing time is shorter. In the other class of plastic bonding process an epoxyethane resin is applied to a metal at 100.degree.-120.degree. C. by stroking with a rod or as a powder to cold metal by a flame-spray gun, followed in both cases by an oven cure cycle of one to two hours at approximately 180.degree. C. or for a shorter time at a higher temperature.
In both of the preceding methods surface preparation is important to the formation of a good bond and the existence of a uniform and consistent oxide layer also promotes good bonding. Therefore, after degreasing with both solvent degreasers and alkaline cleaing agents, the metal is treated with a chromic acid-sulfuric acid pickling solution. A chromic acid anodizing process may also be used to prepare the oxide for bonding.
Presssure can be applied either in a suitable press having heated platens or in an autoclave using a rubber blanket. When thin gauge materials are being bonded vacuum bagging techniques can be used to apply pressure.
Plastic bonding has allowed substantial design flexibility in the aerospace industry by providing for the production of smooth surfaces because no mechanical fasteners are required. It has also provided for lighter and stronger structures because stress concentrations which accompany fastener holes are eliminated and load transfer from one structural member to another is less abrupt thereby providing for more favorable stress distribution throughout a structure assembled by plastic bonding. The ability to use plastic bonding techniques to join structural members of aircraft has also reduced the corrosion succeptibility of such structures because the use of such a technique inherently eliminates traps or other unsealed and inaccessible pockets where corrosion could go undetected. Nothwithstanding the obvious advantages of plastic bonding procedures,the ability of such a procedure to be successfully put into practice to produce complex shapes using the methods and apparatus of the prior art would require complex and expensive tooling and a complex and inefficient bagging operation.